This invention relates to coupled cavity travelling wave tubes.
A typical coupled cavity travelling wave tube as at present known will be described with reference to FIG. 1 of the accompanying drawings which represents a longitudinal section through the slow wave structure of the tube.
The slow wave structure consists of a generally cylindrical tube 1 which is divided by partitions such as 2, 3, 4 and 5 into a series of cavities such as 6, 7 and 8.
Along the axis 9 of the tube a beam-hole is provided in each partition 2, 3, 4 and 5 so as to permit the passage of an electron beam from the input end 10 of the slow wave structure to the outward end 11 of the slow wave structure. Each beam hole is surrounded by a drift tube referenced 12, 13, 14 and 15 respectively in the case of the beam holes in the partitions 2, 3, 4 and 5. Between the end of one drift tube and the beginning of the next drift tube along the axis of the tube is an interaction gap referenced 16 in the case of the interaction gap between drift tubes 12 and 13; 17 in the case of the interaction gap between the drift tubes 13 and 14; and 18 in the case of the interaction gap between the drift tubes 14 and 15. As shown the interaction gaps 16, 17 and 18 are disposed symmetrically about the transverse central plane of the respective cavity 6, 7 and 8.
The electron beam 19 passed in operation along the axis 9 of the slow wave structure in the direction of the arrow is, as represented, cylindrical in cross-section. Ihis electron beam interacts with the electric field of an electro-magnetic wave which is propagated along the structure with a phase velocity approximately equal to the velocity of the electrons in the beam 19. The series of cavities such as 6, 7 and 8, resonant at microwave frequencies, are inter-connected such that the phase shift between adjacent cavities is determined by the frequency of the aforementioned electro-magnetic wave. The use of short drift tubes such as 12, 13, 14 and 15 separated by interaction gaps 16, 17 and 18 is to maximise the interaction between the electron beam and the electro-magnetic wave propagating along the structure. This form of coupled cavity travelling wave tube is commonly referred to as a "space harmonic" tube.
Although with this example, and as mentioned, each interaction gap is symmetrically disposed about the transverse central plane of its cavity, it is known to offset the interaction gaps towards the output end of the tube with the object of compensating for the reduction in the mean velocity of the electron beam in that region.
The type of slow wave structure illustrated in FIG. 1 acts as a bandpass filter whose characteristics may be represented by a dispersion diagram as shown in FIG. 2 of the accompanying drawings. This diagram shows the relationship between the frequency of the signal and the phase shift per cavity. Because the slow wave structure is periodic in space the diagram is repeated periodically in the horizontal direction. The reason for this is that the electrons of the beam respond to the instantaneous phase of the electric field as they pass through the interaction gaps and the electrons are indifferent to changes of phase by integral multiples of 360.degree.. FIG. 2 also shows a line representing the velocity of the electrons in the beam. It will be seen that the aforementioned line lies close to the dispersion curve over an appreciable band of frequencies in the second space harmonic of the electro-magnetic wave on the structure. Over this band of frequencies the interaction between the electrons in the beam and the electro-magnetic wave results in a net transfer of energy from the beam to the wave with resultant r.f gain.
The operation of the tube may be understood by considering an electron which passes through the center A of interaction gap 16 in FIG. 1 at the moment when the field in the gap is at a maximum and tending to accelerate the electron. If the frequency of the wave on the structure is such that the wave is synchronous with the velocity of the electrons as shown in FIG. 2 then the field in interaction gap 17 will also be a maximum in the forward direction when the electron reaches the center B of interaction gap 17. Conversely, an electron which passes through the center B ofinteraction gap 16 when the field is maximum and retarding will be progressively slowed down. This process ensures that the electrons tend to become bunched as they travel down the tube with the faster electrons tending to catch up with the slower electrons. These bunches induce currents in the cavities losing energy in the process and this energy is transferred to the electro-magnetic wave on the structure.
The strength of the interaction between the electron beam and the slow wave structure is not uniform across the passband of the structure. In particular it varies inversely with the group velocity of the wave on the structure and therefore tends to very large values at the band edges. The strong interaction between the beam and the structure at the band edges can result in oscillations close to the band edge frequencies. Not only are such oscillations a source of unwanted r.f output from the tube but also in some cases such oscillations can generate sufficient power to destroy the tube. Commonly such oscillations are encountered at the upper cut off frequency of the structure where the phase shift per cavity is 360.degree..
One object of the present invention is to provide an improved coupled cavity travelling wave tube in which the tendency to oscillate at the band edge is reduced.